Information is transmitted in an optical channel using optical modulation. In a receiver, the received optical signal contains the information, which could have been modulated in amplitude, phase, polarization or a combination thereof. The demodulation of the optical signal is done using a transducer, e.g. photodiode, that converts from the optical to the electrical domain. The transducer delivers an electrical signal, which is processed to extract the information contained in the optical signal. The maximum to minimum optical power, i.e. dynamic range, together with the transducer define the received electrical signal dynamic range. Modern communication systems use complex modulation schemes, e.g. quadrature modulation, to increase the communication channel efficiency. The efficiency of complex modulation schemes is proportional to the maximum operation frequency, i.e. baud rate, and dynamic range.
The received optical signal is transformed to an electrical signal using a transducer. Most transducers convert optical signals to electrical current. However, the current magnitude, which is proportional to the received optical power, needs to be amplified. Typically, during amplification the electrical current is converted to a voltage. The current-to-voltage amplifier, or transimpedance amplifier, together with the bias circuits for proper operation remains one of the most challenging components in an optical receiver.
Incoming optical signal transformed to the electrical domain contain a variable component, i.e. the information, and a fix component, i.e. direct current (DC). The DC component must be minimized so that its effect is negligible in the receiver. Typically a sink circuit, i.e. a DC cancellation circuit, is used at the TIA input. If the receiver architecture uses differential circuit topologies for better performance, then the receiver will be sensitive to unbalanced DC inputs, i.e. input offset. Moreover, offset within the receiver will also be generated due to asymmetry existing in the receiver. In a balanced receiver, each of the differential inputs may have a transducer, therefore, the sink circuit must be able to remove the constant component while minimizing the difference between the fixed components, i.e. offset between the input signals
Receiver circuits are designed to provide large gain, e.g. more than 70 dB of transimpedance gain. Therefore, any offset introduced at the input is amplified accordingly, resulting in the partial or complete loss of information. Offset may also be generated within the receiver as a consequence of a mismatch and a process variation during chip fabrication, and similar to the offset introduced by the transducer, the information can be partially or completely lost if there is no offset correction. A DC cancellation circuit and an offset cancellation circuit may be used, whereby each of these components independently sense a reference and inject a corrective signal. Furthermore, conventional sensing and correction are typically done in the high frequency (HF) path, consequently, the HF performance may be affected in bandwidth, noise, linearity or a combination thereof.
An object of the present invention is to overcome the shortcomings of the prior art by providing a receiver including a DC cancellation circuit that minimizes the effect of DC components in the incoming electrical signals, and an embedded offset cancellation circuit for minimizing the effect of the offset introduced by receivers with differential topologies.